The invention concerns a method of MR (=magnetic resonance), in particular NMR (=Nuclear Magnetic Resonance), MRT (=Magnet Resonance Tomography), MRI (=MR imaging) and/or spatially resolved MR spectroscopy by means of an MR tomograph, in which a sample introduced in a measurement volume in an external magnetic field is first excited by an excitation pulse and the signal formed by the transverse magnetization thus produced is then read out after a time interval by a receiving coil.
Such a method is known from Hahn, E. L., Maxwell, D. E., 1952. Spin echo measurements of nuclear spin coupling in molecules. Physical Review 88, 1070 (=reference [5]).
This invention concerns a method of magnetic resonance (NMR=Nuclear Magnetic Resonance or MRT=Magnetic Resonance Tomography) in which a single excitation pulse is used to generate refocused magnetization (‘spin echo’) at an instant TE (time echo) after the pulse. An echo after a single pulse, termed ‘edge echo,’ was observed as early as 1955 (see references [1] and [7]), however the experimental conditions described therein are unsuitable for practical application. The ‘single pulse echo’ appears there only as a small secondary signal due to the technically associated imperfections of a rectangular pulse used for conventional excitation.
Also known from the literature (see, for example, references [7a] to [7d]) are so-called ‘delayed focus’ pulses, which conceptually result from the superposition of the excitation and refocusing pulse required to produce a spin echo, so that the spin echo is formed at an instant that is shorter than the duration of the pulse.
In contrast thereto, the inventive method described in more detail below is based on specifically phase- and amplitude-modulated excitation pulses for flexible and efficient production of self-refocusing transverse magnetization.
Whereas, with this class of methods, the measuring signal contained in the spin echo is not read out until a corresponding time delay has elapsed after the excitation pulse, with other MRT measuring methods, a completely different time sequence is possible. Reference [14], for example, describes a method in which the signal is read out intermittently during a radio-frequency pulse used for excitation. This is achieved by dividing the excitation pulse into a grid of very short single pulses. This is therefore a technically wholly different MRT measuring method from the spin echo method described above, which is the point of departure for this invention.
Magnetic resonance, referred to hereinafter as MR, is based on measurement of the transverse magnetization of a sample, which is introduced into an external magnetic field. ‘Sample’ is used here as a generic term and can mean a measuring sample to be analyzed or, in animal experiments, an experimental animal or, in human applications, a test-person or patient.
Nuclear spins contained in the sample are excited by a radio-frequency pulse (=RF) using transmit coils placed in the examination volume. The excited spins precess according to the Larmor condition at a frequency ω, which is proportional to the magnetic field B0 at the position of the spin:ω=γB0.  (1)where γ is the gyromagnetic ratio. The object of the measurement (using receiving coils) is the signal located in the region of the sensitive volume of the receiving coils, which is formed as the sum of the transverse magnetization of the excited spins. Due to inhomogeneities of the magnetic field across the examination volume, spins have different resonance frequencies at different locations.ω=ω0±δω,  (2)where δω is referred to as the off-resonance frequency. Signals of spins each having a defined resonance frequency ω are termed isochromates. In dependence on the off-resonance frequency δω, a dephasing of the signal components of different isochromates occurs, i.e., the amplitude of the measured signal (called free induction decay (=FID)) decreases. Dephasing as a function of the Larmor frequency ω corresponds to the slope dφ/dω of the phase φ(ω,t)=ωt+φ0(ω) of the isochromates. Where φ0(ω) is the phase after excitation. During FID, dφ/dω constantly increases and, after an adequate duration of free precession, is always positive.
By contrast, reference [15] describes a method for producing transverse magnetization that exhibits a constant phase.
To simplify the description without restricting its general applicability, a rotating reference system can be assumed such that ω0≡0.
The signal decay caused by the dephasing described above is characterized by the decay constant T2*. For measurements in MR tomography, T2* is typically in the range of 5 to 100 ms. If a realistic excitation pulse with a finite duration is used—in accordance with the excitation profile of the pulse—the isochromates are already dephased by the end of the pulse. For simple, symmetrical pulses with a simple profile (Gauss, Sinc, etc.) used in MR tomography, the dephasing accumulated during the pulse corresponds to the free dephasing across half of the pulse duration.
The local field inhomogeneity is caused by the technical inhomogeneities of the magnet used and by different susceptibility properties of the materials or tissue contained in the sample. The measurement of T2* can therefore provide important information about the tissue properties. The decay constant T2* is given by 1/T2*=1/T2+1/T2′, where the transverse relaxation time T2 results from magnetic field fluctuations and T2′ results from static magnetic field inhomogeneities. T2′ decay is not desirable for the measurement of other parameters such as T2, proton density and the longitudinal relaxation T1.
Dephased isochromates can be rephased by spin echo formation (see reference [5]). Here, a refocusing pulse is used at an instant t=TE/2, which inverts the accumulated phase of the isochromates. As the phase develops, all isochromates are rephased according to their respective frequency at echo time TE and a spin echo is formed. This refocusing is only possible for the static component T2′; refocussing of the T2 decay is fundamentally impossible.
Spin echo formation is one of the most important measuring methods in MR tomography, especially in the form of the so-called TSE (turbo spin echo) technique (originally RARE (Rapid Acquisition with Relaxation Enhancement), also termed FSE (=Fast Spin Echo)) (see reference [6]), which is based on the formation of multiple spin echoes by repeated refocusing. Combined pulses, which consist of the excitation pulse and the refocusing pulse, will be referred to hereinafter as Hahn pulses.
Spin echo formation by means of Hahn pulses is, however, also subject to restrictions. The application of a refocusing pulse requires a certain minimum time (typically 1 to 5 ms when MR tomography is used on humans), and the minimum echo time is therefore limited. Moreover, the z-magnetization present due to the refocusing pulse is also inverted. In the case of methods commonly used in MR tomography, which are based on multiple excitations with a repetition time TR (=Time Repetition), signal saturation occurs, which becomes stronger the shorter TR is in comparison with the longitudinal relaxation time T1. Because T1 is in the range of 0.5 to 2 s for biological tissue, spin echo experiments are usually performed with a TR of approx. 0.5 to 10 s.
MR tomography, also known as magnetic resonance imaging (=MRI), MR imaging, or magnetic resonance tomography (=MRT), is a non-invasive method, which makes it possible to resolve and represent the inner structure of objects, spatially in three dimensions. It is based on the energy behavior of atomic nuclei in a magnetic field, which permits excitation of their nuclear spins by means of suitable radio-frequency pulses and subsequent analysis of the reaction. MRT imaging is used above all in medicine, in order to obtain views of the interior of the human body.
The signal of the atomic nuclei of the object under examination, which is irradiated as a reaction to the excitation with radio-frequency pulses, is read out with suitable receiving coils. The spatial encoding required to be able to allocate the measurement signal to a location within the object to be imaged is performed by additional, spatially varying, magnetic fields Bz(x,y,z,t), which are superimposed on the static main magnetic field B0 and cause the atomic nuclei to have different Larmor frequencies at different locations. Conventionally, magnetic fields with as linear a change of strength as possible along the respective spatial direction, so-called constant or linear magnetic field gradients, are used for this purpose. Conventional gradient systems generate three orthogonal gradients in the x-, y- and z-direction. However, local gradient systems are also used in spatial coding. 1-, 2- or 3-dimensional spatial encoding is applied by varying the magnetic field gradient in all three spatial directions in accordance with known principles, i.e. Fourier encoding, filtered back projection, or another known method.
Time-variable magnetic field gradients and RF pulses are superimposed on a steady-state magnetic field in order to produce a signal that can be used for MRT.
The object of this invention is therefore to present an MR method of the type described in the introduction that achieves the following:                1) Production of a spin echo by means of a single pulse, which prepares the spin system in such a way that spontaneous self-refocusing arises.        2) Production of a spin echo having a variable echo time with no lower limit.        3) Production of a spin echo by means of a pulse having a small flip angle, such that signal saturation is reduced in periodic application of the pulse with short repetition times.        4) Production of a spin echo that is further away from the end of the pulse than the length of the pulse itself.        5) A longer echo time in the case of short pulses and a limited B1 amplitude.        